The data, which is based on a user’s browsing history, is shared with third-party companies to help advertisers target customers.

The use of personalised data is the subject of a complaint filed under the EU’s GDPR legislation.

Dr Johnny Ryan from web browser Brave, Jim Killock at Open Rights Group and Michael Veale of University College London submitted the complaint in September, claiming the use of the data is unlawful.

But the evidence filed today with the national data protection authorities in the UK, Ireland and Poland details how sensitive the data in user profiles can be.

The complaint also criticises the Interactive Advertising Bureau (IAB), the body that develops industry standards for online advertising.

IAB’s list of categories includes one marked ‘incest/abuse support’, which the complainants said could enable ad auction companies to target and profile an internet browser as an incest or abuse victim.

“Ad auction systems are obscure by design”, said Katarzyna Szymielewicz, president of Panoptykon Foundation, which filed the complaint in Poland.

“Lack of transparency makes it impossible for users to exercise their rights under GDPR. There is no way to verify, correct or delete marketing categories that have been assigned to us, even though we are talking about our personal data.”

The complaint states the data is not necessary for effective advertising and is calling for it to be excluded from ad auctions.

The evidence comes a week after the French data regulator handed down a €50m fine to Google for breaching EU data protection rules.

City A.M. understands Google disputes the claim as the categories refer to content on the webpage rather than a user's browsing habits.

A spokesperson for Google said: “We have strict policies that prohibit advertisers on our platforms from targeting individuals on the basis of sensitive categories such as race, sexual orientation, health conditions, pregnancy status, etc.

“If we found ads on any of our platforms that were violating our policies and attempting to use sensitive interest categories to target ads to users, we would take immediate action.”